Ceramic substrates with highly conductive metal vias

ABSTRACT

A ceramic substrate is provided with filled via holes for electrical or thermal feedthrough to or from an electronic device, each via hole having at least one dimension from about 4 to about 50 mils. The via holes are filled with copper, silver or gold for high conductivity and the filling is without visible voids at a magnification of 1000 diameters. The filling is preferably hermetic and is achieved by first electrodepositing metal into the holes and extending therefrom as dumbbell shaped plugs and then heating the substrate to the melting point of the metal so that metal from the dumbbell ends enters the connecting portion within the via hole to seal any aperture therein.

This application is a division of application Ser. No. 07/902,082, filedJun. 22, 1992 now U.S. Pat. No. 5,340,947.

BACKGROUND OF THE INVENTION

This invention relates to a ceramic substrate for a packaged electronicdevice and particularly to via holes through the substrate filled with ahighly conductive metal to provide electrical and/or thermal feedthroughto or from the device. Most particularly it relates to a ceramicsubstrate with filled via holes which provide hermetic sealing to apackaged electronic device.

For some purposes, it is essential that electronic devices employingmicrocircuitry utilize conducting paths as short as possible to minimizeinductance losses in the circuitry. It is also essential that theconducting paths be highly conductive to minimize resistance losses andconsequent heat generation.

It is also essential for some purposes, particularly involvingrelatively high amperage and prolonged continuous use, that heat betransferred away from the electronic devices in a quick and effectivemanner.

Finally, it is also essential for some purposes, that the aforementionednecessary electrical and/or thermal conductivities be obtained withoutthe sacrifice of hermetic sealing in the package because moisture andother constituents of ambient air can be detrimental to sensitivemicrocircuitry and because some devices are intended to operate in evenmore hostile environments.

To deal with these problems, the prior art has gone to via holes inceramic substrates, filled with a conductive metal.

U.S. Pat. No. 4,942,076, granted Jul. 17, 1990 to Ramachandra M. P.Panicker et al. discloses a method for making metal filled via holes ina ceramic substrate by squeegeeing a tungsten paste containing a binderinto the via holes in the substrate, sintering to burn off the pastebinder to leave a porous tungsten mass in each via hole, squeegeeing acopper paste on the top of the sintered tungsten in the via holes andreflowing the copper in the paste into the pores of the sinteredtungsten. It is disclosed that the amount of binder in the tungstenpaste (and thereby the amount of porosity of the sintered tungsten) canbe adjusted so that the thermal coefficient of expansion of thetungsten/copper composite matches that of the ceramic substrate toprovide stability against separation with temperature changes.

U.S. Pat. No. 4,861,641, granted Aug. 29, 1989 to Brian C. Foster et al.discloses a method for making a fired ceramic substrate with metallizedvias by punching via holes in a green tape, filling the punched holeswith a tungsten ink and then firing the green tape with its filled viaholes. To obtain hermeticity, the co-fired tungsten-filled vias are thenfurther treated by depositing another metal thereon by brush coating andheating or by electroless deposition; and this further treatment isrepeated, as necessary, until hermeticity is achieved.

U.S. Pat. No. 4,732,780, granted Mar. 22, 1988 to Stephan P. Mitoff etal. discloses producing a hermetic feedthrough in a ceramic substrate byproviding a sheet of liquid phase sinterable ceramic composition havinga feedthrough hole, filling the hole with refractory metal metallizationmaterial, firing the resulting structure to produce a sintered substratewith adherent metallization comprising refractory metal and glass andthen contacting the refractory metal with an electrically conductiveintrusion metal and heating the resulting structure so that theintrusion metal melts and displaces the glass.

U.S. Pat. No. 4,131,516, granted Dec. 26, 1978 to Peter Bakos et al.discloses a ceramic substrate with via holes which are first primed witha palladium coating on the inside surfaces of the holes, then coatedwith an iron film onto the palladium and finally filled with copperflowed into the holes by melting. The iron film is disclosed asessential in order to achieve good adhesion between the copper and thepalladium.

U.S. Pat. No. 5,113,315, granted May 12, 1992 to Michael L. Capp, et al.discloses, inter alia, via holes filled with copper for the purpose ofthermal conductivity to pass generated heat to heat sink structures. Thecopper in the via holes, deposited initially by electroless coating andthereafter by electrodeposition is shown in the drawings as completelyfilling the via holes; and grossly it does fill the holes. However, thefilling is not complete because, inherently, complete filling of a holecannot be obtained by electrodeposition. In electrodeposition, metal canbe deposited onto a cathode only as long as the cathode surface isavailable to the electrolyte solution. Once the hole is almost closedand the flow of electrolyte solution is substantially blocked,electrodeposition cannot continue and the hole remains less than totallyfilled.

U.S. Pat. No. 5,100,714, granted Mar. 31, 1992 to Kalman F. Zsambokydiscloses the metallizing of a ceramic substrate, which may contain viaholes, by electroless coating followed by electrodeposition and finallyby a heating step to a temperature just below the melting temperature ofthe electrodeposited metal, specifically at the melting point of theeutectic composition of the metal and its oxide. In the case ofelectrodeposited copper, which melts at 1085.4° C., the heating step iscarried out at a temperature between about 1066° C. and 1075° C. At thistemperature, the copper within the via hole does not melt and the holeremaining after electrodeposition is not filled.

Those of the foregoing patents which deal with hermetic sealing achievetheir results by complex and expensive procedures and, in some cases,with composite filler materials which have poorer electrical and thermalconductivities than copper, silver, gold or alloys thereof.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention there is produceda substrate for providing electrical or thermal feedthrough to or froman electronic device in a package comprising a thin ceramic plate havinga plurality of via holes therethrough, each of said via holes having atleast one transverse dimension measuring from about 3 mils to about 50mils, each of said via holes being completely filled with a metalselected from the group consisting of copper, silver, gold and alloysthereof, said filling being in the form of a dumbbell-shaped plug ofsaid metal, said dumbbell shape comprising a narrow, connecting centerportion filling said hole and said two wider end portions extendingbeyond the ends of said via hole.

The ceramic plate may comprise any ceramic material which is strong,thermally stable and a good insulator. Aluminum oxide is most commonlyused because of its excellent qualities and low cost. Where improvedthermal conductivity is desired, beryllium oxide or aluminum nitride maybe used.

Copper is generally the preferred metal filler for the via holes byreason of its excellent electrical and thermal conductivity and lowcost. For convenience, the invention will be described hereinafter,primarily in terms of its preferred embodiment, namely, embodying analuminum oxide substrate with metallic copper as the filler in the viaholes.

In another embodiment, there is produced a substrate for providingelectrical or thermal feedthrough to or from an electronic device in apackage comprised of this ceramic plate having a plurality of via holestherethrough, each of said via holes having at least one transversedimension measuring from about 3 mils to about 50 mils, each of said viaholes being completely filled with a metal selected from the groupconsisting of copper, silver, gold and alloys thereof, said fillingproviding hermetic sealing to said package so that helium gas at ambienttemperature and at a pressure differential of one atmosphere does notpass therethrough into the package at a rate as high as 1×10⁻⁹ cc/sec.after the substrate has been subjected to 250 heat/cool cycles between-50° C. and 150° C.

In still another embodiment, the invention embodies a process ofproviding electrical or thermal feedthrough to or from a thin ceramicplate which comprises producing in said plate a plurality of via holes,each having at least one transverse dimension from about 3 mils to about50 mils, applying to the entire surface of said substrate a thin coatingof a conductive metal, applying a resist coating to each of the majorsurfaces of said plate except at a small area around each entry of eachvia hole, electrodepositing a conductive metal of the group consistingof copper, silver, gold and alloys thereof onto said plate to the extentthat each of said via holes is partially, but not completely filled withsaid metal and to the extent there is a deposit of said metal in each ofsaid small areas which is sufficiently thick to fill at least about 5percent of the volume of said via hole, and preferably at least about 50percent, removing said resist coating and said thin conductive metalcoating where it is exposed and thereafter heating said ceramic plate toa temperature at least as high as the melting point of saidelectrodeposited metal whereby metal from at least one of said smallareas at each entry of each via hole is drawn into said via hole tocomplete the filling thereof.

When the amount of electrodeposited metal in each via hole is relativelysmall (i.e. approaching about 5 volume percent), the small area aroundeach via hole must be correspondingly larger so that there is enoughelectrodeposited metal at each end to fill the remainder of the holeafter melting.

The via holes in the ceramic plate may be produced therein either whenthe ceramic material is in the "green" sheet form (i.e., before it isfired), or in its final, hardened solid form (after firing).

When holes are punched into the ceramic in the green sheet, theirplacement in the final fired sheet may not be as precise as desired. Onthe other hand, holes which are fired after being created by punchinghave stable side walls to which metallic deposits may be stronglyadhered.

When the via holes are produced in the final fired ceramic substrate,generally by laser action, the side walls of the via holes may bestructurally weak. Any metals coated onto such side walls cannot adherewith any strength greater than the coherent strength of the side walls,themselves.

In accordance with this invention, it is preferable, when the via holeshave been produced in the hard, fired ceramic, to preheat the substratebefore the electroless metal deposition step, to a temperature highenough to produce incipient fusion on the walls of the via holes. Whenthe substrate is alumina, the temperature is from about 1200° C. and1550° C. and preferably from about 1400° C. to about 1450° C. Preferablythe temperature is raised to the incipient fusion level over a period offrom about 4 to about 40 hours, held at the incipient fusion level for aperiod from about 1 to about 3 hours and then permitted to cool toambient temperature over a period from about 4 to about 40 hours.

Copper and alumina have different coefficients of thermal expansion; andit might be expected that there would be a tendency for the materials toseparate after repeated thermal cycles at least to the extent whichwould impair hermeticity. It has been found, however, that when thewalls of the via holes are sufficiently close, the total expansiondifference between the two materials is not so great as to produceforces sufficient to overcome the strong adhesive forces generated bythe process of this invention. For via holes of circular cross section,adequate adhesion is obtained when the circle diameters are in the rangefrom about 3 mils to about 50 mils. When the via holes are elongated intransverse cross-section, the adhesion along the elongated surfaces issufficient to overcome the forces generated by the expansion differenceeven in the long direction, provided that the short dimension is in therange from about 3 mils to about 50 mils.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a substrate in accordance with the invention ina typical embodiment in which the via holes are circular incross-section parallel to the major surfaces of the substrate;

FIG. 2 is a plan view similar to that of FIG. 1 but where the via holesare elongated in cross-section parallel to the major surfaces of thesubstrate;

FIG. 3 is a fragmentary enlarged cross-section of a single via hole in asubstrate prior to any metallization thereof;

FIG. 4 is a fragmentary enlarged cross-section similar to FIG. 3 butafter a thin metal deposition step;

FIG. 5 is a fragmentary enlarged cross-sectional view similar to FIG. 4but after the deposition of a resist layer in areas not close to the viaholes;

FIG. 6 is a fragmentary enlarged cross-sectional view similar to FIG. 5but after the electrolytic deposition of a relatively thick layer ofcopper in areas not covered by a resist layer;

FIG. 7 is a fragmentary enlarged cross-sectional view similar to claim 6but after removal of the resist layer and the electroless metal layerthereunder;

FIG. 8 is a fragmentary enlarged cross-sectional view similar to FIG. 7but after the substrate has been heated to a temperature high enough tomelt the electrolytically deposited copper;

FIG. 9 is a fragmentary enlarged cross-sectional view similar to FIG. 8but after lapping to remove deposited copper extending beyond thesurfaces of the substrate;

FIG. 10 is a fragmentary enlarged cross-sectional via hole similar toFIG. 3 except that the diameter of the hole is larger at transversecross-sections at each surface than at a cross-section within theinterior of the via hole; and

FIG. 11 is a photomicrograph at 200× enlargement of a transfer sectionof a filled via hole in an alumina substrate where copper is the solefilling material first by electroless coating, then by electrodepositionand finally by melting.

FIG. 12 is a photomicrograph similar to that of FIG. 11, except that itis at 1000× enlargement.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the description hereinbelow, the successive steps in the process ofthe invention are shown in successive FIGS. 3-9 in which similarelements are designated by the same reference numerals.

A typical ceramic substrate in accordance with this invention is shownin plan view in FIG. 1. In FIG. 1, ceramic body 11 is made of adielectric, insulating tough material, such as alumina, beryllia oraluminum nitride. It is thin, as compared to its length and breadth,typically from about 5 to about 100 mils thick and preferably from about10 to about 40 mils thick. It contains a plurality of filled via holes12 such as those shown in FIGS. 8 or 9, each of which is circular intransverse cross-section.

In FIG. 1, there are 72 filled via holes arranged in a pattern of twoconcentric squares. The precise arrangement of the filled via holes inany particular substrate is dependent on the circuitry in the device itis intended to serve.

FIG. 2 is generally similar to FIG. 1, except that the via holes areelongated in transverse cross-section, rather than circular. Suchelongated via holes are particularly useful for thermal transfer. FIG. 2shows the elongated via holes to be elongated linearly. It is to beunderstood, however, that the via holes may be elongated arcuately, orzigzaggedly or in any other shape as long as the width of the via holes(the distance between opposite walls) is between about 3 mils and about50 mils.

FIG. 3 shows, in cross-section, a single via hole of circularcross-section in a ceramic substrate prior to any metallization thereof.In FIG. 3, hole 14 is shown within ceramic body 11. As shown, hole 14 istypical of a laser-fabricated via hole in that it has a somewhat largerdiameter at one surface (the upper surface in the Figure). This is thedirection from which the laser device has been applied.

The walls of hole 14 are shown in FIG. 3 to be smooth; and,macroscopically, they are smooth. However, the walls of laser-fabricatedholes, may be seen, microscopically, to be fragmented and weak. Asdisclosed above, the ceramic substrate when laser-fabricated, ispreferably subjected to a firing step to the temperature of incipientfusion, which firing leaves it unchanged in general appearance from whatis shown in FIG. 3, but more receptive to adherent metal coating thereonby electroless deposition.

FIG. 4 shows, in cross-section, the same ceramic substrate and same viahole as in FIG. 3, but after the deposition of a thin layer of metal 16.The thickness of layer 16 in FIG. 4 and in subsequent FIGS. 5-9 is notto scale and is actually very thin, typically only about 0.02 to about0.05 mils in thickness.

The thin layer deposition may be by any method known in the art for suchdeposition, including vapor deposition. However, the preferred method isby electroless deposition.

Electroless metal deposition processes, including pre-cleaning steps andsurface activation steps, are known in the art. Typical methods aredisclosed in U.S. Pat. Nos. 3,993,799; 4,087,586; 4,220,678; 4,136,216;4,199,623; 4,259,113; 3,958,048 and 5,058,799. The metal deposited bythe electroless process is generally copper, sometimes deposited over anextremely thin activation layer of palladium. However, the electrolessdeposit may constitute metals other than copper, such as cobalt, iron,nickel, gold, silver and manganese.

FIG. 5 is similar to FIG. 4, except that it shows the deposit of aresist layer 17 in a pattern which covers the major surfaces of thesubstrate except for small areas around each via hole. The resist layeris very thin and is not shown to scale in FIGS. 5 and 6. The resistlayer is initially deposited over the entire surface of the substrateand contains a photosensitive material. The surface is then shaded in apattern to cover the area around each via hole, exposed to light tocross-link the exposed portions and then subjected to a solvent toremove the unexposed resist material around each via hole.

FIG. 6 is similar to FIG. 5, except that it shows an electrolyticdeposit 18 of copper, or other highly conductive metal onto theelectroless deposit in the areas not covered by the resist material 17,namely onto the walls of the via hole and onto the major surfaces of thesubstrate adjacent each end of the via hole. When electrolyticdeposition is carried out as far as it will go, each via hole will besubstantially filled with electrolytically deposited copper, except thata small hole 19 will remain because electrolytic deposition ceases whenfresh electrolyte can no longer flow to the cathode. Hole 19 is actuallysmaller than shown in FIGS. 6 and 7 when the electrolytic depositproceeds as far as it can; and in those cases, hole 19 is microscopic indiameter.

The electrolytic deposition need not proceed, however, until its naturalcessation. It may be stopped at any time after at least about 5 volumepercent of the via hole is filled with electrolytically depositedcopper. Preferably at least 50 volume percent of the hole is filled.

The electrolytically deposited copper in and around each via hole isessentially dumbbell-shaped with the copper within the via holeconstituting a connecting center portion and the copper deposited beyondthe major surfaces of the substrate constituting the dumbbell ends.Preferably, the combined volume of the ends is from about 100% to about400% of the volume of the center portion, with larger end volumes beingassociated with less metal filling in the via holes.

FIG. 7 is generally similar to FIG. 6, except that the substrate hasbeen treated to remove the resist layer and the electroless depositedlayer from those areas which are not protected by the electrolyticallydeposited copper.

FIG. 8 is similar to FIG. 7, except that it shows the substrate after ithas been heated to above the melting point of copper, permitting moltencopper to flow into and fill hole 19 by capillarity and/or gravity andthereby close the hole. FIG. 8 shows a convexity at each end of thedumbbell-shaped plug by reason of melting the copper therefrom. In somecases, the convex dumbbell ends may be substantially unequal, indicatingthat most, or even all, of the metal flow into the via hole may be fromonly one end of the dumbbell. It is believed that small temperaturedifferences within the heating oven may cause one end of the dumbbell toreach the melting point of the copper before the other and may thereforecause most of the copper to flow from the hotter end.

The shape of the ends of the via plug after the melting step may varyfrom what is shown in FIG. 8. For example, one side may be concave,rather than convex by reason of capillary action. However, the plug isdumbbell-shaped in the sense that each of the two ends is wider incross-section than the connecting portion within the via hole.

When the via hole is in the state shown in FIG. 8, it is generallyhermetically sealed to the extent that when the substrate is inposition, closing an electronic package, helium gas at ambienttemperature and at a pressure differential of one atmosphere does notpass therethrough into the package at a rate as high as 1×10⁻⁹ cc/sec.after the substrate has been subjected to 250 heat/cool cycles between-50° C. to 150° C.

For many purposes, the substrate, as shown in FIG. 8 may be the finalproduct with electrical and/or thermal connections being made to thedumbbell ends of the plug. For some purposes, it may be necessary, ordesirable, to remove the dumbbell ends by lapping and leave the via holefilling flush with the major surfaces of the substrate, as shown in FIG.9.

Where hermetic sealing is essential, the lapped product of FIG. 9 maynot be as reliable as the unlapped product of FIG. 8. Any lapped productwhich does not pass the helium hermeticity test may, of course, be usedfor any purpose which does not require hermeticity. However, ifhermeticity is essential, the lapped product of FIG. 9 may be reworkedthrough the series of steps described above in connection with FIGS. 4to 8, and then subjected to a final lapping step, as shown in FIG. 9.

FIG. 10 is similar to FIG. 3 in that it shows a substrate prior to anymetal deposition thereon. It differs from FIG. 3 in that the via hole iswider at each end than at a center portion at a level between the twomajor surfaces of the substrate. Such via holes may be obtained byapplying laser heating at each hole from both major surfaces, eithersimultaneously or successively. The narrower waist in each filled viahole provides a mechanical lock helping to anchor each plug in place.

FIG. 11 is a photomicrograph of a transverse section of a circular viahole at 200× enlargement. In FIG. 11, the dark outer portion of thephotomicrograph is the alumina substrate and the light-colored circularportion is an all-copper filling (copper having been used as both thethin metal coating of FIG. 4 and the electrodeposited layer of FIG. 6).The section was made after the substrate had been subjected to 1200thermal cycles between -50° C. and 150° C. The dark areas within thecopper filling are not voids but localized areas of copper oxide; andthe diagonal line at about 9 o'clock is a polishing artifact.

FIG. 12 is a photomicrograph similar to that of FIG. 11, except that itis at 1000× enlargement and shows only a portion of the circumference ofthe plug. As in FIG. 11, the dark areas within the copper filling arenot voids, but copper oxide. There are actually no visible voids in thecopper filling at 1000× enlargement.

FIG. 12 shows that the copper extends to the filling-alumina interfaceand actually infiltrates the alumina wall at microscopic fissurestherein.

While the invention has been described with respect to the preferredembodiments, it will be understood by those skilled in the art thatother modifications and embodiments fall within the purview of thisinvention.

What is claimed:
 1. The process of providing electrical or thermalfeedthrough to or from a thin ceramic plate which comprises producing insaid plate a plurality of via holes, each having at least one transversedimension from about 3 mils to about 50 mils, applying to the entiresurface of said substrate a thin coating of a conductive metal, applyinga resist coating to each of the major surfaces of said plate except at asmall area around each entry of each via hole, electrodepositing aconductive metal of the group consisting of copper, silver, gold andalloys thereof onto said plate to the extent that each of said via holesis partially, but not completely filled with said metal and to theextent there is a deposit of said metal in each of said small areaswhich is thick enough to fill at least 5% of the volume of said viahole, removing said resist coating and said thin coating where it isexposed and thereafter heating said ceramic plate to a temperature atleast as high as the melting point of said metal whereby metal from atleast one of said small areas at each entry of each via hole is drawninto said via hole to complete the filling thereof.
 2. The process ofclaim 1 wherein said electrodeposited metal is thick enough to fill atleast 50% of the volume of said via hole.
 3. The process of claim 1wherein said thin coating of conductive metal is applied by electrolessdeposition.
 4. The process of claim 1 wherein said metal is copper andsaid plate comprises alumina.
 5. The process of claim 1 wherein saidmetal is copper and said plate comprises beryllia.
 6. The method ofclaim 1 wherein said metal is copper and said plate comprises aluminumnitride.
 7. The process of claim 1 wherein said via holes are producedby punching holes through a ceramic plate precursor in green sheet formand thereafter firing said precursor to produce a hard and tough ceramicplate.
 8. The process of claim 1 wherein said via holes are produced bylaser action on a hard and tough ceramic plate and said plate isthereafter heated, prior to said electroless coating step to atemperature which produces incipient fusion at the walls of said viaholes.
 9. The process of claim 8 wherein said plate comprises aluminaand said incipient fusion heating step is to a temperature from about1400° C. to about 1450° C.